A spark plug for providing ignition in an internal combustion engine, such as a gasoline engine, has the following structure: an insulator is provided externally of a center electrode; a metallic shell (main metal fitting) is provided externally of the insulator; and a ground electrode which forms a spark discharge gap in cooperation with the center electrode is attached to the metallic shell. The metallic shell is generally formed from an iron-based material, such as carbon steel, and, in many cases, plating is performed on its surface for corrosion protection. A known technique for performing such plating forms a plating layer having a 2-layer structure consisting of an Ni plating layer and a chromate layer (Japanese Patent Application Laid-Open (kokai) No. 2002-184552, “Patent Document 1”).